1. Field of the Invention
This invention relates to a rock bit having a sintered bushing such as disclosed in U.S. Pat. No. 4,207,658 of common assignee with the present invention and, more particularly, to a rock bit having such a bushing with a relatively thin wall.
2. Description of the Prior Art
As disclosed in the above-identified patent, the sintered bushing of the rock bit comprises an alloy powder compressed into the desired shape providing a porous matrix and then sintered to provide an integral, porous bushing. The pores are infiltrated with an anti-galling material to provide a relatively long-lasting rock bit bearing. A preferred anti-galling material is a silver alloy which, because of its silver content, is relatively expensive. It has thus been determined that a substantial savings could be realized if a thin-walled bushing, having the porous matrix with the silver alloy anti-galling material infiltrated therein, could be substituted for the relatively thick-walled bushing presently available.
As presently fabricated, the thick-walled porous bushing for a rock bit has a wall thickness of approximately 0.180 inches providing the bearing with substantial rigidity. Such bushing is pressed into a mating opening in the cutter cone having a slightly smaller diameter to provide an interference fit therebetween. With this interference, the bushing is stationarily retained in the cone.
In a thin-walled bushing, under the dynamic loading to which such bushing is subjected when the bit is drilling, there is sufficient deformation in the wall that the bearing tends to "walk out" (i.e. move axially outwardly) and also to rotate or turn with respect to the cutter cone or adjacent backing ring. Further, it is believed that any greater interference will cause buckling in the thin wall. Thus, the relative movement of the bushing cannot be prevented by merely increasing the interference fit. (For the most part the thin-walled bushing has a wall thickness of 0.045 inches, however, wall thicknesses up to 0.125 inches still encounter the above problems.) Therefore, to retain the thin-walled bushing stationarily seated within either a backing ring or directly within a mating opening in the rolling cutter requires an interference fit, within the acceptable range, in addition to other structure which assists in maintaining the thin-walled bushing stationary in the cutter.